1. Field of the Invention
The present invention relates to wireless data communication; and, in particular, the present invention relates to network access protocols used in wireless data communication.
2. Background of the Invention
In a wireless communication network, all participants in a channel share the radio channel as a network resource. In order to have a high utilization rate, and to ensure that all participants are given a fair access to the channel, various network access protocols have been developed. Typically, the more sophisticated a channel access protocol is, the higher the channel utilization can be achieved. However, the more sophisticated a channel access protocol is, the more sophisticated transceivers and software are required to implement that protocol. As a result, the proper choice of a channel access protocol involves optimizing a scarce resource to some cost constraints.
Among wireless channel access protocols, the ALOHA protocol is amongst the simplest. Under ALOHA, each participant transmits over the channel as soon as it has a message to send. The recipient of the message sends an acknowledgement message upon successfully receiving the transmitted message. The sender of the first message waits for this acknowledgement message, and if this acknowledgement message is not received after a predetermined time period, the sender assumes that the first message was not successfully received and retransmits the first message. Because each participant transmits at will, a collision occurs whenever two or more participants attempt to transmit simultaneously. Thus, under ALOHA, the expected maximum channel utilization is 18% for a large number of channel participants.
A higher channel utilization rate can be achieved by collision avoidance. One channel access protocol that implements collision avoidance is the Carrier Sense Multiple Access (CSMA) protocol. Under the basic CSMA protocol, a transmitting station having a message to send over the common channel first "listens" for activity on the channel. If activity is detected (i.e. another participant is transmitting), then the transmitting station "backs off" for a time period before re-attempting the transmission. The main advantage of the basic CSMA protocol is the reduction of the need for retransmissions, since collisions are rare. CSMA allows channel utilization to reach as much as 50%. However, while CSMA results in a higher channel utilization, CSMA requires more sophisticated hardware to implement.
The channel access protocol in a wireless data network can be determinative of the total bandwidth of the wireless data network. For example, in a cellular digital packet data (CDPD) system, a large number of cellularized base stations are distributed all over the service area. Cellularizing the service area offers two advantages: (i) allow mobile units and base stations to transmit at higher data rate with relatively low power, since the expected distance between a mobile unit and a base station in the vicinity is short; and (ii) larger capacity is provided because base stations which are separated by large enough distances can use the same radio channels. Such a system provides thus very high capacity, low response delay and allows the mobile units to transmit at relatively high data rates. In such a system, two-way symmetrical and reliable data links can also be provided. However, such a system is expensive and complex.
In a CDPD system, because connectivity is maintained over the entire duration of a data communication session, multiple channels must be provided to allow multiple sessions to be maintained simultaneously. To locate a recipient mobile unit of a message, the network broadcasts the address of the recipient mobile unit from all the base stations in the service area until the recipient unit responds. Thus, a large amount of network resources is dedicated to locating mobile units. Further, to maintain continuous connectivity and to allow real time performance, when the session is established, the CDPD unit is associated with a base station with which it communicates. In addition, because a mobile unit can be expected to be used in a moving vehicle, it is possible that the mobile unit moves out of the service range of the initially associated base station and moves into the service range or ranges of one or more such base stations during the duration of a session. Thus, provisions must be made to disengage an associated base station and to engage an additional base station or stations ("hand off") during the course of the session. The control mechanisms for maintaining a CDPD session, including tasks typically termed "connectivity management" and "mobility management", involve sophisticated algorithms which require high performance computers to handle setting up the session, maintaining the session, and tracking the communicating mobile units as they move between service areas of the cellularized base stations. The complexity of the system requires a large investment in expensive equipment. Often, these control mechanisms are centralized, i.e. a large network switching or control center is provided to handle the mobile units in a given service area, so that, at times of heavy data traffic, the network control center may become a bottle neck, introducing undesirable latency into the system.
Another major disadvantage of the CDPD system is the requirement that the receiver of the mobile unit must be on at all times to receive messages. As a result, such a mobile unit requires a battery that is, at the present time, too undesirably bulky for mobile use. It would also be extremely difficult for power-saving features to be provided in such a mobile unit. Unlike a pager, which monitors only one paging channel, the mobile unit of a CDPD system must monitor a different radio channel when it is located in a different cell. Further, control information (e.g. timing information and recipient addresses) are broadcast by base stations in dedicated control channels. While a pager can be assigned a periodically occurring time-slot during which it "wakes" up to check for messages, a similar scheme in a mobile unit of a CDPD system would require timing in all the cellular base stations to be synchronized. Failure to synchronize all base station can lead to a mobile unit checking for the broadcast of its address during the wrong time slot.